The present invention relates to methods of making large diameter domes for tanks such as liquid fuel rocket tanks and, in particular, methods of manufacture of such domes from undersized blanks.
Advancements in satellites, and the enhancements to telecommunications and other services that sophisticated satellites make possible, have dramatically increased the number of commercial satellites being launched. As each launch is an expensive event, there has been a trend toward increasing the number of features and components on satellites, allowing multiple users to share the costs and benefits of the satellite launch. Increased features have resulted in an increase in the size of these satellites, and consequently the size of the launch vehicles required to carry these satellites into their operational orbit. Generally, satellite launch vehicles are multi-stage rockets, with each stage including its own fuel tank, comprising a cylindrical body having a domed leading end. It is in the manufacture of these larger rocket domes that current manufacturing methods fall short.
Rocket domes have typically been fabricated from single blanks of aluminum alloys or other alloys that are hot spun over mandrels to form the desired shape. Common mandrel spin forming methods include clamping a blank between a rotatable spindle and a die, or mandrel, corresponding to the shape to be formed. The clamped assembly is then rotated and the blank is heated while a tool, such as a spinning roller, is used to apply pressure, progressively, to a small area of the metal blank, thereby causing the small area to deform in the direction of the pressure. As the metal blank rotates, a circumferential band of the blank is progressively deformed and, by moving the spinning roller in a radial direction with respect to the rotating metal blank, the desired shape is produced.
Traditionally, rocket domes have been manufactured by the mandrel spinning process from blanks having a surface area that is greater than or equal to the surface area of the domes to be spun. This process results in a dome having the desired diameter and a substantially constant material thickness. Traditional hot mandrel spinning methods have been effective, as long as the surface area of the necessary blank was smaller than the largest commercially available blank diameter. However, to manufacture domes for large rockets using traditional mandrel spinning techniques would require a circular blank with an outside diameter well in excess of the maximum commercially available blank size (currently about 209 inch/5.31 m width plate from the Alcoa mill in Davenport, Iowa). One attempt to solve this problem is seen in U.S. Pat. No. 6,006,569 to Shrayer, et al.
Despite these prior art systems, there remains a need in the art for a less costly and more robust way to fabricate the end domes for large diameter tanks.
The present invention provides a method for forming large diameter hemispheric domes particularly useful for the leading end of launch vehicle fuel tanks. The method includes joining together by friction stir welding plates that are smaller than the largest premium sizes available, and spinning the formed blank. The friction stir welded joint line has material properties that are very similar to the surrounding base material, and thus the stresses imposed on the blank during the spin forming process do not create cracks or tearing. This is particularly true if the blank is first annealed.
An exemplary method of forming large hemispheric domes comprises providing at least two plates of material having abutting edges, friction stir welding the two plates together along the abutting edges to form a blank, and spin forming the blank into a dome. An alternative method according to the present invention includes providing at least two plates of material having abutting edges, welding the two plates together along the abutting edges to form a blank, and simultaneously spin forming and stretching the blank into a dome.
The two starting plates may be rectilinear, and the method further includes cutting the blank into a circle prior to the spin forming step. Alternatively, there may be four plates in the shape of squares that are friction stir welded together to make a larger square. Preferably the plates are at least 1 inch (2.5 cm) thick.
Desirably, the method includes heating the blank during the spin forming step, preferably to between 500xc2x0 F. and 700xc2x0 F. (260-371xc2x0 C.). Furthermore, preferred method steps are annealing the blank prior to the spin forming step, and solution heat treating the dome. If the plates are Al 2219, then the annealing is done at about 775xc2x0 F. (413xc2x0 C.) for about 3 hours and then furnace cooling at 500xc2x0 F. and then in the ambient air, and the solution heat treating is to T62 temper at 995xc2x0 F. (535xc2x0 C.).
A further aspect of the invention is placing a material that retards grain growth between the abutting edges of the plates prior to the friction stir welding step. For example, the material that retards grain growth may be an aluminum-scandium alloy.